1 Infrared Limb Sounding With Cassini CIRS: Optimal Viewing Strategy Using Horizon Nodes Conor A. Nixon, Richard K. Achterberg, Department of Astronomy University of Maryland College Park, MD 20742 301-286-6757 301-286-1550 [email protected] [email protected] F. Michael Flasar Solar System Exploration Branch NASA Goddard Space Flight Center Greenbelt, MD 20771 301-286-3071 [email protected] Abstract—In this paper1 2 we investigate a question of science optimization during Cassini flybys of Titan. The Composite Infrared Spectrometer (CIRS) makes limb observations – along an atmospheric path above surface – during the closest approach period when the visible horizon circle is moving swiftly across the planet. We have sought to discover if any points on the horizon are preferred for limb sounding due to having minimum movement relative to the surface. By numerical calculation, backed by geometric analysis, we find that two limited regions on the horizon are continuously visible during the entire encounter. We term these ‘limb nodes’ and show how they may be employed by CIRS to optimize science by minimizing the source of systematic error due to spatial smear. These conclusions are applicable to many similar scenarios of spacecraft limb sounding during hyperbolic flyby encounters. TABLE OF CONTENTS 1. INTRODUCTION .................................................................1!2. LIMB VIEWING WITH CASSINI CIRS ..............................2!3. LIMB VIEWING HORIZONS AND NODES...........................4!4. ANALYTICAL TREATMENT ...............................................7!5. IMPLICATIONS FOR OPERATIONS..................................11!6. SUMMARY AND CONCLUSIONS .......................................12!ACKNOWLEDGEMENTS ......................................................12!REFERENCES.......................................................................13!BIOGRAPHY ........................................................................13!1. INTRODUCTION Since July 2004, the Cassini spacecraft has been orbiting Saturn, making more than 60 close flybys of the large satellite Titan, the only moon in the solar system to possess 1 978-1-4244-3888-4/10/$25.00 ©2010 IEEE. 2 IEEEAC paper #1174, Version 5, Updated January 4, 2010 a substantial atmosphere. During these close approaches, Cassini’s Composite Infrared Spectrometer (CIRS) instrument is used to sense the atmosphere in limb-sounding mode – viewing along a path that does not does intersect the surface – to measure the vertical variation of temperature, aerosols (hazes) and gas composition. The opacity of a limb path, defined as: € χν≡ kνρd∫l (1) [where kν is the spectrally-dependent absorption co-efficient (cm2/g), ρ is the atmospheric density (g/cm3), and dl is the path element (cm)] is usually sharply peaked at the tangent point, so long as the emissions are non-saturated (optically thin, χ<1) and therefore the retrieved values at various altitudes correspond to the atmospheric column above a specific latitude and longitude co-ordinate on the surface. When viewing the limb, the available choice of latitude and longitude locations is defined by the instantaneous horizon circle. At large distances (D) an entire hemisphere is visible, and the horizon is a great circle (circumference) of the globe. However as the spacecraft approaches to distances where the range is no longer much greater than the planetary radius (R), the horizon circle rapidly shrinks and expands around closest approach, and its center (the sub-spacecraft track) also moves across the surface. Obtaining each limb sounding profile near closest approach – when vertical resolution becomes better than an atmospheric scale height (the vertical distance in which density drops by 1/e) – requires a time that is not insignificant compared to the movement of the projected horizon circle on Titan’s surface. Therefore, a question of optimization arises: is there a preferred point (or points) on the horizon to view, where the lateral movement of the tangent point is least, so that the vertical profile obtained most closely corresponds to a single latitude and longitude?2 In this paper we investigate the problem both numerically and theoretically, showing that optimum solutions exist. Our recommendations, described in the context of CIRS sounding of Titan are expected to be of broad applicability to other remote sensing applications. 2. LIMB VIEWING WITH CASSINI CIRS Cassini Mission and Flybys As Cassini orbits Saturn, it makes frequent encounters with the giant moon Titan, serving the dual purposes of enabling Titan science investigations – including remote sensing, in situ sensing of fields and particles, radar sensing, gravity measurements and radio occultations, and more - but also as a driver for the overall Saturn tour, using Titan’s gravity to deflect the spacecraft’s path to rendezvous with smaller satellites and to change the inclination to the ring plane. Table 1 – Cassini Mission Phases Mission Phase Start Time End Time Saturn Orbits Titan Flybys Prime Mission 1-JUL-2004 30-JUN-2008 75 45 Equinox Mission 1-JUL-2008 30-SEP-2010 63 26 Solstice Mission 1-OCT-2010 16-SEP-2017 155 56 The mission has three major phases: the prime mission (PM), referring to the first four years of operation as originally planned; the extended or equinox mission (XM or EM) covering the next 27 months and including the northern spring equinox of Saturn and Titan in August 2009; and the second extended or solstice mission (XXM or SM) stretching a further seven years (as currently envisioned at time of writing) beyond the end of the EM, and including the northern summer solstice of the Saturnian system in May 2017. Further details of the Cassini mission phases can be found in Table 1. CIRS Instrument The Cassini Composite Infrared Spectrometer (CIRS) is a hybrid instrument, with two separate interferometers sharing a common telescope, fore-optics, scan mechanism, reference laser, and back-end electronics. See [1] for design details. The